The invention relates to a soundboard of composite fibre material construction comprising at least one composite fibre laminate for use for an acoustic musical instrument, particularly a bowed stringed instrument.
The invention will be described in greater detail below using the example of the soundboards of bowed stringed instruments. However, it can also be used for other acoustic musical instruments (such as guitars and pianos) which are provided with a resonant body or resonant back-plate.
The resonant body of a bowed stringed instrument is formed by the two soundboards (top plate and back plate) and the ribs which connect them. The top plate is made in the traditional way from spruce, and the back plate is generally made from maple.
In recent years attempts have also been made to produce the soundboards of acoustic musical instruments in composite fibre material construction. Structures of composite fibre material construction generally consist of long fibres which are preferably oriented in certain directions and a carrier or matrix material which is generally a thermosetting or thermoplastic plastics material.
The previous efforts to produce soundboards of composite fibre material construction intended for acoustic musical instruments are aimed without exception at copying as well as possible the acoustic characteristics of the wood for which a composite fibre material is to be substituted. Examples of these attempts in the previously known prior art are provided for instance by DE 37 38 459 A1, EP 0 433 430 B1, U.S. Pat. Nos. 5,895,872 and 5,905,219. Thus DE 37 38 459 A1 aims at xe2x80x9ca macroscopic heterogeneity almost equal to the woodxe2x80x9d and states as the object that xe2x80x9cthe composite materialxe2x80x9d should xe2x80x9chave similar characteristics to sprucexe2x80x9d.
An unsatisfactory feature of these previously known soundboards of composite fibre material construction appears to be that from the acoustic point of view they are equivalent to but in no way superior to very good solid wood soundboards of traditional construction.
The object of the invention, therefore, is to create a soundboard of composite fibre material construction which has a perceptibly better acoustic quality by comparison with excellent soundboards of traditional construction. In particular the soundboard according to the invention should have substantially higher radiated power whilst retaining the usual and desirable timbre of a solid wood soundboard.
This object is achieved according to the invention by the combination of the following features:
a) at least one test strip cut out of the soundboard has a quality quotient (QM=cL/rho) of at least 0.02 m4/sg, preferably at least 0.04 m4/sg, where cL is the velocity of sound (in m/s) of the longitudinal waves in the longitudinal direction of the test strip and rho is the average total density (in g/m3) of the test strip;
b) the area of the soundboard defined by the outline of the soundboard is chosen to be of such a size that
b1) the frequency of the main body resonance (B1 mode) of bowed stringed instruments lies within the following ranges:
in the violin between 480 and 580 Hz, preferably between 510 and 550 Hz,
in the viola between 380 and 500 Hz, preferably between 420 and 460 Hz,
in the cello between 150 and 210 Hz, preferably between 170 and 190 Hz,
in the double bass between 80 and 120 Hz, preferably between 90 and 110 Hz,
b2) the frequency of the second-lowest body resonance (0,0 mode) in the guitar lies between 180 and 240 Hz, preferably between 190 and 220 Hz,
b3) the frequency of the lowest resonance (0,0 mode) of the piano or grand piano soundboard lies between 40 and 60 Hz, preferably between 45 and 55 Hz.
In detail, the invention is based upon the following considerations and tests:
If a test strip is cut out of a soundboard (as will be explained in detail below in the description of an embodiment), then the acoustic quality of this test strip can be assessed using a quality quotient QM which is defined as follows:
QM=cL/rho
In this case cL is the velocity of sound (in m/s) of the longitudinal waves in the longitudinal direction of the test strip and rho is the average total density (in g/m3) of the test strip.
Thus the quality quotient rises the greater the velocity of sound of the longitudinal waves is in relation to the vibrating mass. Thus a high value of QM corresponds to a favourable ratio of stiffness to mass of the soundboard.
In the case of spruce wood cL=5800 m/s and rho=400 kg/m3 results in a typical quality quotient QM=0.0145 m4/sg. In the tests on which the invention is based, the highest achievable value with resonant spruce wood was measured at QM=0.016 m4/sg. This value corresponds to the values occurring in the soundboards of the most famous violin makers (such as Antonio Stradivari). Below-average resonant spruce wood lies at QM=0.012 m4/sg.
By contrast, with test strips from soundboards of composite fibre material construction it is possible to establish quality quotients of more than 0.06 m4/sg. Thus the acoustic material quality of soundboards of composite fibre material construction is almost four times as high as the acoustic material quality of the best resonant spruce wood which has aged over a long time. In spite of this well known fact, however, it has not been possible hitherto to create soundboards of composite fibre material construction which having regard to all necessary aspects are superior to the solid wood soundboards. The reasons for this difficulty and the sense of the combination of features according to the invention are apparent from the following considerations.
If a soundboard in composite fibre material construction is produced with the same geometric dimensions as a soundboard made of wood, then because of the substantially higher quality quotient QM much higher characteristic frequencies (resonant frequencies) are produced. This rise in the characteristic frequencies leads to an undesirably sharp or nasal tone and thus changes the timbres of the instrument quite detrimentally.
It might then be thought that the excessively high characteristic frequencies of a soundboard of composite fibre material composition could be lowered (and shifted again in the direction of the characteristic frequencies of a conventional solid wood soundboard) by dimensioning the soundboard of composite fibre material construction so that it is thinner than a corresponding solid wood soundboard. However, in the tests on which the invention is based it was shown that the quality quotient QM of a soundboard of composite fibre material constructionxe2x80x94in total contrast to the quality quotient of a conventional solid wood soundboardxe2x80x94is dependent upon thickness, as a reduction in the board thickness in fact results simultaneously in a reduction in the quality quotient QM. Thus if the thickness of a soundboard of composite fibre material construction is reduced (in order to lower the resonant frequencies again into the desired range), then the quality quotient QM is also reduced with it and thus the acoustic advantage which the composite fibre material construction has per se over the traditional wooden construction is lost.
With these considerations as a starting point, therefore, the invention follows a fundamentally different route in order to place the resonant frequencies of a soundboard of composite fibre material construction into the desired range which is usual for solid wood soundboards.
In the solution according to the invention, the raising of the characteristic frequency due to the composite fibre material construction (with which the very desirable increase in the quality quotient QM is increased) is compensated for by such a geometry-induced lowering of the characteristic frequency by which the quality quotient QM is not significantly lowered. According to the invention, for this purpose the area of the soundboard is of greater dimensions than in a soundboard made from solid wood for a bowed stringed instrument of the same timbre. An increase in area of the soundboard results in a shifting of the characteristic frequencies downwards. Because of its greater area the soundboard can then be given a greater thickness without the characteristic frequencies going above the necessary range for the desired and usual timbre. Thus the resulting quality quotient QM lies markedly above that of a thinner plate which is not enlarged and is of composite fibre material construction.
Since an enlargement of the vibrating area simultaneously results in an increase in the sound radiation and thus an increase in the acoustic efficiency of the instrument, in the solution according to the invention not only is the desired timbre of the classical bowed stringed instruments achieved but also the additional tonal characteristics such as xe2x80x9cprojectionxe2x80x9d, xe2x80x9cvolumexe2x80x9d and xe2x80x9cdynamicsxe2x80x9d are improved. Thus the soundboard according to the invention enables instruments to be built which correspond to the conventional instruments made from solid wood as regards the hearing habits (sensing the timbre) but which are markedly superior to the traditional instruments as regards their acoustic efficiency.
If in the case of a soundboard made from solid wood for a conventional bowed stringed instrument the area of the soundboards were to be enlarged, then this would shift the characteristic frequencies of the instrument so far downwards that a hollow (xe2x80x9cdullxe2x80x9d) timbre would result. In a conventional bowed stringed instrument with solid wood soundboards, because of the low cross-stiffness of the solid wood boards a widening of the boards would also lead to the formation of modes of vibration with narrow parallel antiphase antinodes which result in a low sound radiation due to hydrodynamic short-circuits (cf. Cremer, Lothar: xe2x80x9cPhysik der Geigexe2x80x9d, Stuttgart 1981, page 341).
Therefore an increase in the area of the soundboard is only sensible when a material (such as a composite fibre material) is used which by comparison with wood has a higher bending strength and consequently a higher velocity of sound.
The acoustic condition formulated serves for controlling comparable timbres. The condition relates to the frequency of the main body resonance whichxe2x80x94according to the relevant literaturexe2x80x94is designated as B1 mode. The second lowest body resonance is referred to for the guitar and is designated as 0,0 mode. The lowest resonance of the soundboard of pianos or grand pianos, which according to its vibrational shape is likewise designated as 0,0 mode.
The said resonances, particularly the respective typical vibrational shape thereof, are explained in greater detail in relation to the embodiments described below.
In the tests on which the invention is based, modal analysis of outstanding instruments from famous violin makers (such as Antonio Stradivari or Guarneri del Gesu) were carried out in the inventor""s acoustic laboratory. In violins of which the timbres are assessed by artists and trained listeners as pleasant and balanced the B1 mode always lies in a relatively narrow frequency band between 510 and 550 Hz. A violin with a B1 mode markedly above this frequency range tends to sound harsh and sharp, whereas a violin with a B1 mode below this frequency range tends to have a hollow and dull timbre. The characteristic frequency of the B1 mode can therefore be considered as a reliable acoustic indicator for the timbre of a bowed stringed instrument.
These and further details of the invention (for instance obtaining, measurement and evaluation of test strips) are explained in greater detail below with reference to the drawings.